Polyolefins, such as polyethylene or polypropylene, are widely used in a number of everyday articles, machines, consumer goods, and the like. Polyolefins are relatively inexpensive to produce and are capable of providing a number of useful functions. Polyolefins may be formed into various shapes, films, laminates, and the like. Polyolefins may be coated on, or co-extruded with various substrates. Polyolefins may also be combined with other materials to form a structure having a plurality of layers, each layer having a specific purpose. Laminates, for example, may comprise a plurality of layers, such as a configurationally rigid core layer, an outer liquid-tight layer, an oxygen gas barrier such as a mid-layer of aluminum foil, and/or other layers depending on application needs. However, polyolefins may be too rigid and hard for the target applications, or cannot be extended without tear, or difficult to process due to their high viscosity at the processing temperature, or become brittle at colder temperatures due to their relatively high glass transition temperatures. These properties may render various polyolefins brittle, hard, inflexible, and thus unsuitable for particular uses, particularly uses at lower temperatures or may lead to slower processing speeds and/or excessive rejects during processing. Many applications of polyolefins would benefit from a polyolefin having useful properties over a wide range of temperatures, and under a variety of conditions. It would also be beneficial if the viscosity of melts could be reduced affording better and higher speed processability in common polymer processing plants, such as extruders, melt blowers, etc. Such useful properties may include both high- and low-temperature performance in the areas of impact strength, toughness, flexibility, and the like. The ability to adjust the stiffness-toughness balance and processability of polyolefins is important to meeting the needs of a broad range of applications at a lower cost and thus to expanding the potential of polyolefins in delivering desired performance at a reduced cost.
In some instances, the stiffness-toughness balance may be tailored by changing the molecular structure of polymers or by changing their composition (i.e. making copolymers), stereoregularity, molecular weight, etc. The stiffness-toughness balance may also be readily shifted by making blends of polymers with different stiffness and toughness or by blending polymers and plasticizing agents, such as polyolefin fluids and low molecular weight polymers, particularly low glass transition temperature polyolefin fluids and polymers. A plasticizer added to high molecular weight, highly crystalline stiff polyolefins softens the structure to improve the toughness of such materials. Plasticizers with low glass transition temperature also extend the flexibility of plastics to lower temperatures by lowering the glass transition temperature of the polymer-plasticizer blend. Plasticizers are also beneficial during polymer processing due to improvements in a number of characteristics, such as lubricity, viscosity, ease of fusion, etc. The concept of plasticization, the benefits of using plasticizers, and the different methods of using plasticizers are discussed in detail in J. K. Sears, J. R. Darby, THE TECHNOLOGY OF PLASTICIZERS, Wiley, New York, 1982. Although this monograph focuses on the plasticization of poly(vinyl chloride), PVC, the concepts and benefits are analogous to those applicable to plasticized polyolefins.
Since the plasticizers are often fluids at ambient temperature, plasticized polymer blends are sometimes also referred to herein as fluid-enhanced polyolefins or fluid-enhanced polymers. These plasticized polymers may be made by a variety of methods. Plasticized polyolefins are traditionally made by starting with polyolefin polymers and plasticizer fluids made in separate plants. Since the polymer is in its essentially pure, fully recovered solid state, it is difficult and expensive to blend it with a plasticizer fluid. A flexible but expensive process of making them starts with the high molecular weight polymer resin component in its solid, essentially pure, fully recovered state. In one prior art process, the one or more solid polymers are first melted and then blended with the plasticizing fluid or low molecular weight resin, which is commonly referred to as a melt-blending process. In another prior art process, the one or more solid polymers are put into solution with a suitable solvent before being blended with the plasticizing fluid or low molecular weight resin, which is commonly referred to as an off-line solution blending process. Off-line blending to produce plasticized polyolefins has numerous issues in that it increase processing cost. For example, melt blending is difficult requiring high-performance blenders or extruders due to the high viscosity of polymers and the low viscosity of the plasticizing fluid. Off-line solution blending is also an expensive process due to the need for redissolving the polymer and possibly the plasticizer blend components and also due to the cost of solvent handling. Also, as mentioned before, handling plasticizers requires additional special equipment.
As described above, polymer blends may be made by a variety of ways. A flexible but expensive off-line process of making polymer blends uses solid polymers as starting materials, typically outside the polymerization process that produced the polymer blend components. The polymer blend components are typically first melted or dissolved in a solvent and then blended. These processes are known as melt-blending and off-line solution blending, respectively. In melt blending, the solid, often pelletized or baled, polymer blend components are first melted and then blended together in their molten state. One of the difficulties presented by melt blending is the high viscosity of molten polymers, which makes blending of two or more polymers difficult and often imperfect on the molecular level. In solution off-line blending, the solid, often pelletized, polymer blend components are first dissolved in a suitable solvent to form a polymer solution, and then two or more polymer solutions are blended together. After blending, solution blending requires the extraction of solvent from the blend and drying of the blended polymer. Solution blending can overcome the viscosity issue associated with melt blending, but is expensive due to the need for redissolving the polymer blend components and due to the cost of solvent handling.
The common feature of both melt blending and off-line solution blending is that the polymer blending components are made in separate plants and the solid polymers then are reprocessed either in a molten or in a dissolved state to prepare the final polymer blend. In fact, these off-line blending processes are often operated by so-called compounders, generally independent of the manufacturers of the polymer blend components. These processes add considerable cost to the cost of the final polymer blend. The production and full polymer recovery in separate plants and subsequent reprocessing increases the costs of producing such blends because of the need for duplicate polymer recovery lines and because of the need for separate blending facilities and the energy associated with their operations. Off-line solution blending also requires extra solvent, and facilities for polymer dissolution and solvent recovery-recycle. Handling plasticizers for blending them with the base polymer represents a further challenge, since plasticizers are typically low molecular weight, low melting point fluids or soft materials. Compounding plants are typically equipped for handling free-flowing pelletized components, thus are not equipped for receiving, storing, and blending fluids and/or soft, baled substances. The disclosed processes blend the plasticizer fluids and/or soft polymers in the polymerization plant producing plasticized polymers for final use, or plasticized-polymer masterbatches for further compounding in stable pelletized forms, thus afford their handling in downstream processing plants without the need for special handling and without the associated investment costs. For the above-outlined reasons, substantial reprocessing costs could be saved if the polymer blends could be made in one integrated polymerization plant in-line, i.e. before the recovery and pelletizing of the solid polymer blend components. In some instances, especially when the plasticizer is liquid at ambient temperature, significant cost savings can also be achieved even if the plasticizer was produced in a separate plant by blending the liquid plasticizer with the polymer in its diluted state, i.e., before the viscosity-reducing components of the polymerization system, such as monomer and the optional inert solvent/diluent, are removed from the product polymer or polymer blend.
The disadvantages of a separate polyolefin blending plant associated with the melt blending and off-line solution blending processes is alleviated with the prior art method of in-line solution blending of polymers using a series reactor configuration. Utilizing the series reactor configuration, product blending may be accomplished in the solution polymerization reactor itself when the effluent of the first solution polymerization reactor is fed into the second reactor operating at different conditions with optionally different catalyst and monomer feed composition. Referring to the two-stage series reactor configuration of FIG. 1, the two different polymers made in the first and second reactor stages are blended in the second stage yielding a blended polymer product leaving the second reactor. Such reactor series configuration may be further expanded into more than a two-stage series configuration (three or more reactors in series). Generally, a series of n reactors may produce a blend with as many as n components or even more present in the effluent of the last reactor. Note that in principle, more than n components may be produced and blended in n reactors by, for example, using more than one catalyst or by utilizing multiple zones operating at different conditions in one or more reactors of the series reactor cascade. While mixing in the downstream reactor(s) provides good product mixing, particularly when the reactors are equipped with mixing devices, e.g., mechanical stirrers, such series reactor configuration and operation presents a number of practical process and product quality control problems due to the close coupling of the reactors in the cascade. One of the most important difficulties in commercial practice is ensuring proper blend and monomer ratios to deliver consistent blend quality. Additional complications arise when the blend components have different monomer compositions, particularly when they have different monomer pools, such as in the case of blending different copolymers or in the case of blending homo- and copolymers. As will be shown later in the detailed description of the disclosed processes, this is often the case in the production of plasticized polyolefins. In such instances, either some monomers need to be converted completely before passing the effluent to the downstream stages or need to be removed between the reactor stages. In many cases, such solutions are not practical. Also, since the monomer streams are blended, there is no option for their separate recovery and recycle mandating costly monomer separations in the monomer recycle lines.
The above-outlined issues with series reactor operations are apparent to those skilled in the art of polymer reactor engineering. These difficulties are particularly significant in polymerization because unlike in small-molecule synthesis, reactor conditions determine not only reactor productivities related to blend ratio, but also product properties related to controlling the quality of the polymer blend components. For example, FIGS. 2 and 3 show how reactor temperature and pressure affect polymer properties of fundamental importance, such as molecular weight (MW) and melting behavior. Surprisingly, we found that monomer conversion in the reactor also influences these critical product; attributes (see FIG. 4). Since in a series reactor cascade, the effluent of an upstream reactor flows into the next downstream member of the reactor cascade, the residence time, catalyst concentration, and monomer composition and thus monomer conversion in the downstream reactor cannot be adjusted independently of the operating conditions (particularly of the flow rate) of the upstream reactor. Because of this close and inherent coupling of operating regimes in the reactors of the series cascade, the correlations depicted in FIGS. 2, 3, and 4 further reduce the controllability, flexibility, and thus the usefulness of the in-line blending method in a series reactor configuration. Ultimately, this greatly reduces the number of blend products that can be made in such a series reactor cascade and makes the blend quality difficult to control.
Applying parallel reactors can overcome the disadvantages related to the direct coupling of the polymerization reactors in an in-line polymer blending applying series reactors. While production flexibility is increased, a parallel reactor arrangement necessitates the installation of blending vessels increasing the cost of the process. A need thus exists for an improved and cost-effective method of in-line blending of polymers and plasticizers to avoid the issues associated with the prior-art methods, such as melt blending and off-line solution blending. More particularly, a need exists for an improved in-line method of blending polymers and plasticizers, especially for an improved in-line method of blending polyolefins and plasticizers, where the residence time, monomer composition, catalyst choice, and catalyst concentration can be independently controlled in the polymer reactor(s) and the optional plasticizer reactor(s) prior to the blending step. There is also a need for a simplified and cost-effective polymer-plasticizer blending process to reduce the number of process steps and the associated investment and operating costs in an integrated polymer and plasticizer production and blending process employing parallel reactor trains for producing the polymer-plasticizer blend components in-line, i.e. without recovering the polymer component(s) in its/(their) solid state. Embodiments of the present invention, which follow, meet these needs.